1. Field of the Invention
The present invention generally relates to a control waveform generating apparatus for an electronic musical instrument, and more particularly to a control waveform generating apparatus in which a control waveform is generated for controlling a volume, a tone color and other characteristics of a musical tone to be varied with time.
2. Prior Art
As a conventional example of the control waveform generating apparatus for the electronic musical instrument, a known envelope waveform generating apparatus is designed to generate an envelope waveform which is rendered to be similar to that of an acoustic musical instrument as much as possible. An example of such envelope waveform generating apparatus has been disclosed in Japanese Patent Application Laid-Open No. 58-181091. This apparatus generates a typical envelope waveform signal ENV (shown in FIG. 9) having a so-called ADSR waveform which includes an attack portion A, a decay portion D, a sustain portion S and a release portion R therein.
Generally, the conventional envelope waveform generating apparatus is constituted so that an increment quantity or a decrement quantity is set for each variable portion such as the attack portion, the decay portion and the release portion and an adding operation is repeatedly performed by use of an increment quantity or a decrement quantity so as to obtain an envelope waveform of which waveform level varies with time. When a key in a keyboard is depressed to generate a key-on signal, the attack portion is immediately formed. When the waveform level of the attack portion has reached a predetermined level, the decay portion is formed, and then the waveform level of the decay portion reaches another predetermined level so that the sustain portion is started to be formed. Thereafter, when the depressed key is released, the release portion is started to be formed. These formations for the envelope waveform is performed sequentially.
In the actual use of the envelope generating apparatus, however, it is sometimes required to obtain a musical tone signal MS having an envelope waveform which rises at a time t.sub.1 which is delayed by a predetermined delay time T.sub.D after a time t.sub.ON at which the key-on signal is generated as shown in FIG. 10, rather than an envelope waveform in which the attack portion thereof starts to rise at the time t.sub.ON.
A conventional method for generating the above musical tone signal MS will be described in FIG. 11. According to this method, the delay time T.sub.D is computed in accordance with the following manner. First, a waveform portion A0 having an extremely small inclination is provided ahead of a waveform portion A1 which corresponds to the attack portion A shown in FIG. 9. This waveform portion A0 rises at the time t.sub.ON and the signal level thereof gradually increases to a signal level of L.sub.IN at the time t.sub.1 when the musical tone is generated. The signal level of L.sub.IN is maintained to such a low level that any sensible musical tones are not generated in the delay time T.sub.D.
The above method, however, is disadvantageous in that an additional and complicated processing step is newly required for setting the delay time T.sub.D, so that the process for generating the envelope waveform becomes burdensome inevitably.
Particularly, the waveform shown in FIG. 10 encounters a difficulty in converting the time into the inclination of the waveform because the delay time T.sub.D is set in terms of the increment quantity which represents the inclination of the waveform portion A0.
Thus, though it may be said as possible to generate the envelope waveform similar to that of the acoustic musical instrument in the conventional envelope waveform generating apparatus, it is impossible to generate arbitrary envelope waveforms other than the envelope waveform of the acoustic musical instrument.